Fingerprint sensing system and method

ABSTRACT

A fingerprint sensing system comprises a sensor array with a plurality of sensing structures and read-out circuitry connectable to each of the sensing structures, and power supply circuitry arranged to provide to the read-out circuitry a substantially constant supply voltage being a difference between a high potential and a low potential. The fingerprint sensing system is configured in such a way that the low potential and the high potential are variable while substantially maintaining the supply voltage, and the read-out circuitry is connectable to each of the sensing structures in such a way that a variation in the low potential and the high potential while substantially maintaining the supply voltage results in a change of the charge carried by a sensing structure connected to the read-out circuitry. The change in charge is indicative of a capacitive coupling between the sensing structure and the finger.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/324,549, filed on Jul. 7, 2014. The disclosure of the aboveapplication is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a fingerprint sensing system and to amethod of operating a fingerprint sensor.

BACKGROUND OF THE INVENTION

Various types of biometric systems are used more and more in order toprovide for increased security and/or enhanced user convenience.

In particular, fingerprint sensing systems have been adopted in, forexample, consumer electronic devices, thanks to their small form factor,high performance and user acceptance.

Among the various available fingerprint sensing principles (such ascapacitive, optical, thermal etc.), capacitive sensing is most commonlyused, in particular in applications where size and power consumption areimportant issues.

All capacitive fingerprint sensors provide a measure indicative of thecapacitance between several sensing structures and a finger placed on ormoved across the surface of the fingerprint sensor.

Some capacitive fingerprint sensors passively read out the capacitancebetween the sensing structures and the finger. This, however, requires arelatively large capacitance. Therefore such passive capacitive sensorsare typically provided with a very thin protective layer covering thesensing structures, which makes such sensors rather sensitive toscratching and/or ESD (electro-static discharge).

U.S. Pat. No. 7,864,992 discloses a fingerprint sensing system in whicha driving signal is injected into the finger by pulsing a conductivestructure arranged in the vicinity of the sensor array and measuring theresulting change of the charge carried by the sensing structures in thesensor array.

Although the fingerprint sensing system according to U.S. Pat. No.7,864,992 provides for an excellent combination of fingerprint imagequality and sensor protection, there appears to be room for improvementfor “difficult” fingers, in particular for dry fingers.

SUMMARY

In view of above-mentioned and other drawbacks of the prior art, it isan object of the present invention to provide an improved fingerprintsensing system, in particular providing for improved sensing offingerprints from “difficult” fingers, such as dry fingers.

According to a first aspect of the present invention, it is thereforeprovided a fingerprint sensing system comprising a sensor array having aplurality of sensing structures each facing a surface of the sensorarray and being arranged to capacitively couple to a finger touching thesurface of the sensor array; and read-out circuitry connectable to eachof the sensing structures for providing sensing signals indicative of achange of a charge carried by each of the sensing structures; and powersupply circuitry arranged to provide to the read-out circuitry asubstantially constant supply voltage being a difference between a highpotential and a low potential, wherein: the fingerprint sensing systemis configured in such a way that the low potential and the highpotential are variable while substantially maintaining the supplyvoltage; and the read-out circuitry is connectable to each of thesensing structures in such a way that a variation in the low potentialand the high potential while substantially maintaining the supplyvoltage results in a change of the charge carried by a sensing structureconnected to the read-out circuitry, the change of the charge carried bythe sensing structure being indicative of a capacitive coupling betweenthe sensing structure and the finger.

The above-mentioned “low potential” will herein sometimes also bereferred to as the reference potential of the sensor array.

It should be noted that the low potential and the high potential aretime-varying in relation to a reference potential of a device comprisingthe fingerprint sensing system. Such a reference potential of the devicemay be referred to as “device ground”, and the time-varying lowpotential may be referred to as “sensor ground”. According to variousembodiments of the present invention, the “sensor ground” may vary inrelation to the “device ground”. The potential of the finger maytypically be at a substantially constant level, at least for therelevant time scale for fingerprint acquisition, in relation to the“device ground”. For instance, the body of the user may actually definethe “device ground” for a portable device that is not connected to someglobal reference potential (like mains ground).

The read-out circuitry may provide analog sensing signals, for examplein the form of voltage levels being indicative of the capacitivecoupling between the finger and the sensing structures comprised in thesensor array.

According to various embodiments, however, the read-out circuitry mayinclude circuitry for converting analog signals to digital signals. Suchcircuitry may include, for example, at least one sample and hold circuitand at least one analog to digital converter circuit.

The present invention is based upon the realization that improvedfingerprint sensing performance can be achieved by allowing thereference potential of the sensor array to “swing” relative to thepotential of the finger instead of forcing the finger potential to varywhile keeping the reference potential of the sensor array (the sensorground) constant.

The present inventors have further realized that a fingerprint sensingsystem in which the reference potential of the sensor array is allowedto “swing” can be designed so that it is capable of reading out highquality fingerprint images without having a conductive structuresupplying a signal to the finger and without reducing the thickness ofthe protective layer.

The absence of a conductive structure (such as a conducting framesurrounding the sensor array) simplifies integration of the fingerprintsensor into various devices, such as mobile phones and computers.Moreover, the design of the fingerprint sensor system can be made lessobtrusive and the finish of the product including the fingerprint sensorsystem can be improved.

In addition, sensing performance can be improved in applications wherethe finger or other parts of the hand of a user contacts a conductivepart of the product in which the fingerprint sensing system is included.Since, in various embodiments of the fingerprint sensing systemaccording to the present invention, fingerprint sensing does not relyupon a varying potential of the finger, the finger can be allowed to begrounded (or at least heavily loaded) by such a conductive part of theproduct. Examples of such applications include mobile phones withexposed electrically conductive parts.

When providing an excitation signal to the finger through a directconductive electrical connection, or in other words galvanically drivingthe finger, the potential difference between the finger surface touchingthe sensor array and the sensing structures in the sensor array may bedifferent for fingers with different electrical properties. Forinstance, the potential difference may be lower for dry fingers,resulting in a “weaker” fingerprint image which may be difficult toanalyze.

In various embodiments of the present invention, the electricalpotential of the finger may be considered to be substantially constantand the reference potential of the sensor array may instead be varied.In such embodiments, the potential difference between the finger surfacetouching the sensor array and the sensing structures in the sensor arraywill instead be substantially the same for fingers with differentelectrical properties. Experiments have verified that this results inimproved fingerprint images for dry fingers.

According to various embodiments of the present invention, the powersupply circuitry may be configured to provide a first time-varyingpotential as the low potential and a second time-varying potential asthe high potential.

In this manner, an excitation signal may be provided to the sensor arrayinstead of to the finger. Since the power supply circuitry mayaccordingly pulse the reference potential of the sensor array (“variablesensor ground”), the amplitude of the excitation signal is not limitedto the supply voltage to the sensor array, but may be considerablyhigher. This would allow for the use of a thicker protective coatingcovering the sensing structures and/or allow for new materials to beused, which provides for a more robust and possibly more visuallyattractive fingerprint sensor.

In some embodiments, excitation signals may be provided to both thesensor array and the finger or to the finger only. An excitation signalmay, for example, be provided to the finger using a finger excitationstructure external to the sensor array and/or using sensing structuresthat are currently not sensing.

According to various embodiments, the read-out circuitry may becontrollable to connect to a first set of sensing structures in such away that the variation of the low potential and the high potentialresults in a variation of a potential of each sensing structure in thefirst set, and the read-out circuitry provides signals indicative of thechange of charge carried by each sensing structure in the first set; andconnect to a second set of sensing structures different from the firstset of sensing structures in such a way that the variation of the lowpotential and the high potential results in a variation of a potentialof each sensing structure in the second set.

Hereby, the occurrence of unwanted coupling between sensor elements thatare sensing and sensor elements that are not sensing can be reduced.This provides for an improved quality of the fingerprint sensing carriedout by the fingerprint sensing system. This is particularly the casewhen sensing and non-sensing sensor elements are arranged next to eachother, in other words, when sensing structures in the second set arearranged adjacent to sensing structures in the first set.

Advantageously, the variation of the potential of each sensing structurein the first set and the variation of the potential of each sensingstructure in the second set may be substantially equal.

Furthermore, the fingerprint sensing system may advantageously comprisedriving circuitry connectable to each of the sensing structures andcontrollable to change a potential of a sensing structure connected tothe driving circuitry. Such driving circuitry may be used to provide theabove-mentioned excitation signal to the finger. The driving circuitrymay also be used to reduce unwanted excitation of the finger resultingfrom the variation of the reference potential of the sensor array.

Accordingly, in various embodiments of the present invention, theread-out circuitry may be configured to provide the sensing signalindicative of the change of charge carried by a first sensing structure;and the driving circuitry may be configured to provide a time-varyingdriving signal to a second sensing structure, the driving signal beingsuch that a potential of the second sensing structure is constant orvaries over time with a peak-to-peak amplitude being lower than apeak-to-peak amplitude of a potential of the first sensing structure.

To achieve this effect, the driving circuitry may, for instance, beconfigured to drive the second sensing structure with a driving signalthat is substantially in anti-phase with the variation of the referencepotential of the sensor array. In embodiments where the variation of thereference potential of the sensor array has a peak-to-peak amplitudethat is greater than the supply voltage to the sensor array, the drivingcircuitry will not be able to fully compensate for the variation in thereference potential using the driving signal. However, also in suchembodiments, the unwanted excitation (capacitive drive) of the fingercan be considerably reduced.

It should be noted that the read-out circuitry and the driving circuitrymay advantageously be combined as pixel circuitry that may becontrollable between a sensing state and a driving state.

The driving circuitry may advantageously be configured to simultaneouslyprovide the time-varying driving signal to a plurality of sensingstructures. For instance, the time-varying driving signal may beprovided to all sensing structures except the currently sensing firstsensing structure and surrounding sensing structures.

In embodiments of the fingerprint sensing system according to thepresent invention, the power supply circuitry may comprise a constantvoltage source that is configured to provide the substantially constantsupply voltage and that is dedicated to supplying power to the sensorarray.

Alternatively, the power supply circuitry may comprise isolationcircuitry having an input side for connection to a voltage source and anoutput side connected to the sensor array, the isolation circuitry beingconfigured to prevent current to flow from the output side to the inputside, to thereby allow for an output potential on the output side beingdifferent from an input potential on the input side.

The isolation circuitry may, for example, comprise at least one diodearranged between the voltage source and the sensor array, wherebydifferent potentials can be achieved on the different sides of thediode.

In one embodiment, the isolation circuitry may be configured to providegalvanic isolation between the voltage source and the read-outcircuitry.

Such isolation circuitry providing galvanic isolation is well-known tothose skilled in the art, and may, for instance, include optocouplers,or circuitry based on one or more (micro) coils and/or one or morecapacitors.

According to various embodiments, the fingerprint sensing system mayfurther comprise processing circuitry connected to the sensor array viaa communication interface for acquiring fingerprint data from the sensorarray.

The fingerprint data may be raw fingerprint image data, or the data mayhave been processed and may then be provided in the form of conditionedimage data, as fingerprint template data or in any other form.

The communication interface may be any suitable communication interface,which may be a parallel interface or a serial interface. One example ofa suitable communication interface may be the SPI-interface (SerialPeripheral Interface).

As was mentioned further above, the sensing signals provided by theread-out circuitry may be analog signals, which may be provided directlyto the communication interface. In such embodiments, the fingerprintsensing system may further comprise external circuitry for convertingthe analog sensing signals to digital signals.

To allow undisturbed acquisition of fingerprint data from the sensorarray in embodiments where the reference potential of the sensor arrayis varied at the same time as fingerprint data is acquired, thefingerprint sensing system according to various embodiments of thepresent invention may advantageously further comprise isolationcircuitry for providing galvanic isolation between the sensor array andthe processing circuitry.

Such isolation circuitry is well-known to those skilled in the art, andmay, for instance, include optocouplers, or circuitry based on one ormore (micro) coils and/or one or more capacitors.

Alternatively or in combination with the above-mentioned isolationcircuitry, the power supply circuitry may be configured to keep each ofthe low potential and the high potential substantially constant duringtime periods when the processing circuitry acquires fingerprint datafrom the sensor array.

By configuring the power supply to keep the reference potential of thesensor array constant (and at substantially the same potential as theground level of the processing circuitry acquiring the fingerprint data)during acquisition, the acquisition can take place without beingdisturbed by the varying reference potential during fingerprint sensingevents. In other words, the reading/acquisition of a completefingerprint image from the sensor array may be divided into differenttime slots during which different activities are permitted andforbidden, respectively. For instance, a sensing time slot may befollowed by an acquisition time slot, before it may be time for asensing time slot again. The number and durations of the time slots maydepend on the size of the sensor array and the buffering capability ofthe sensor array. For a swipe sensor (sometimes also referred to as astrip sensor or a line sensor) a single sensing time slot followed by asignal acquisition time slot may be sufficient, while for a touch sensor(area sensor), it may be necessary with several sensing time slots andseveral acquisition time slots that are interleaved.

According to various embodiments of the present invention, the sensorarray may advantageously comprise excitation signal generating circuitryfor generating a time-varying, in relation to the low potential (sensorground potential), excitation signal for synchronizing operation of theread-out circuitry; and an excitation signal output for output of theexcitation signal from the sensor array.

In embodiments, the excitation signal may be used to control or triggerthe power supply circuitry comprised in the fingerprint sensing systemto provide the above-mentioned time-varying, in relation to a referencepotential of the electronic device in which the fingerprint sensingsystem is included, low potential and high potential to the sensorarray.

In other embodiments, the excitation signal output may be conductivelyconnected to the reference potential of the device in which thefingerprint sensing system is included, thereby forcing the lowpotential and the high potential to vary over time in relation to thereference potential.

The excitation signal may, for instance, be a square wave signal inrelation to the sensor ground potential.

Since the sensor ground potential of the sensor array is variable inrelation to a reference potential of an electronic device including thefingerprint sensing system, tying the excitation signal output to thedevice reference potential will result in the sensor ground potentialswinging up and down in relation to the device reference potential.

Swinging of the sensor ground potential, in relation to the devicereference potential, will cause the sensing structures (pixel plates) ofthe sensor array to also swing up and down in potential in relation tothe device reference potential, and thus also in relation to thepotential of the finger touching the top surface of the sensor array.

Hence, tying the excitation signal output to the device referencepotential will result in a time-varying voltage between the finger andat least selected ones of the sensing structures of the sensor array. Bymeasuring the change in charge carried by a sensing structure resultingfrom a change in voltage between the finger and that sensing structure,a measure indicative of the capacitance of the capacitor formed by thesensing structure, the finger and the dielectric coating on the sensingstructure can be deduced. This measure will also be an indication of thedistance between the finger surface and the sensing structure.

Moreover, the fingerprint sensing system according to variousembodiments of the present invention may advantageously be included inan electronic device, further comprising processing circuitry configuredto acquire a representation of the fingerprint pattern from thefingerprint sensing system; authenticate a user based on therepresentation; and perform at least one user-requested process only ifthe user is authenticated based on the representation.

In embodiments where the sensor array comprises the above-describedexcitation signal generating circuitry and excitation signal output, theexcitation signal output may advantageously be connected to a referencepotential of the electronic device, such as, for example, device groundor a higher reference potential of the electronic device (such as 3.3 V,2.5 V or 1.8 V in relation to the device ground potential).

In these embodiments of the fingerprint sensing system (and of theelectronic device comprising the fingerprint sensing system), allterminals (except the excitation signal output) of the sensor array(fingerprint sensor) will exhibit time-varying potential levels inrelation to the device ground potential. To provide for continuoussupply of power to the sensor array and communication between theprocessing circuitry and the sensor array, the fingerprint sensingsystem may, as was described further above, include isolation circuitry.

In various embodiments, furthermore, the fingerprint sensing system maycomprise an electrically conductive finger contacting structure arrangedadjacent to the sensor array for conductive contact with the user'sfinger when the fingerprint sensing system is in use. Such a fingercontacting structure may, for instance, be provided as a bezel or frame,or one or several strips. As an alternative to connecting the excitationsignal output to a reference potential terminal for the electronicdevice (keeping the excitation signal output at a constant voltage inrelation to device ground), the excitation signal output may beconductively connected to the finger contacting structure. In suchembodiments, the user's finger will be the reference level in relationto which the fingerprint sensor ground will vary. This configuration maybe advantageous since the effect of common mode noise on the deviceground on the fingerprint sensing may be reduced.

According to a second aspect of the present invention, there is provideda method of operating a fingerprint sensor comprising a sensor arrayhaving a plurality of sensor elements, each comprising a sensingstructure facing a surface of the fingerprint sensor, wherein each ofthe sensor elements is configured to provide a signal indicative of acapacitive coupling between the sensing structure and a finger placed onthe surface of the capacitive fingerprint sensor; and a power supplyinterface having a low potential input and a high potential input forproviding power to the sensor array, wherein the method comprises thesteps of: providing a first time-varying potential to the low potentialinput of the power supply interface and a second time-varying potentialto the high potential input of the power supply interface, a differencebetween the second time-varying potential and the first time-varyingpotential being a substantially constant voltage; and acquiring fromeach of the sensor elements, while providing the first time-varyingpotential and the second time-varying potential, the signal indicativeof the capacitive coupling between the sensing structure and the fingerplaced on the surface of the capacitive fingerprint sensor.

The above-mentioned signal acquired from each of the sensor elements maybe acquired from one sensor element at a time or simultaneously fromseveral sensor elements at a time. Acquiring from several sensorelements at the same time allows for faster read out from thefingerprint sensor.

The second time-varying voltage is higher than first time-varyingvoltage, and the difference between the second time-varying voltage andthe first time-varying voltage is the supply voltage. The supply voltagemay, for example, be 3.3 V, 2.5 V or 1.8 V.

According to various embodiments, the step of acquiring may comprise thesteps of controlling each sensor element in a first set of sensorelements in such a way that the variation of the low potential and thehigh potential results in a variation of a potential of the sensingstructure of each sensor element in the first set, and each sensorelement in the first set provides the signal indicative of thecapacitive coupling between the sensing structure and the finger placedon the surface of the fingerprint sensor; and controlling each sensorelement in a second set of sensor elements different from the first setof sensing structures in such a way that the variation of the lowpotential and the high potential results in a variation of a potentialof the sensing structure of each sensor element in the second set. Thesensor elements in the second set do not provide signals indicative ofthe capacitive coupling between those sensing structures and the finger.

Furthermore, the step of acquiring may additionally comprise the step ofcontrolling each sensor element in a third set of sensing elements insuch a way that a potential of the sensing structure of the sensorelement is constant or varies over time with a peak-to-peak amplitudebeing lower than a peak-to-peak amplitude of a potential of the sensingstructure of each sensor element in the first set of sensor elements.

Moreover, the method according to embodiments of the present inventionmay advantageously further comprise the step of providing, through acommunication interface comprised in the fingerprint sensor, a signalindicative of a fingerprint pattern of the finger, the signal indicativeof the fingerprint pattern of the finger further encoding errorcorrection data for enabling error correction.

Further embodiments of, and effects obtained through this second aspectof the present invention are largely analogous to those described abovefor the first aspect of the invention.

According to a third aspect of the present invention, it is provided amethod of operating a fingerprint sensor comprising a sensor arraycomprising a plurality of sensor elements each comprising a sensingstructure facing a surface of the fingerprint sensor, wherein each ofthe sensor elements is configured to provide a signal indicative of acapacitive coupling between the sensing structure and a finger placed onthe surface of the capacitive fingerprint sensor; a power supplyinterface having a low potential input receiving a low potential and ahigh potential input receiving a high potential, for providing power tothe sensor array; excitation signal generating circuitry for generatinga time-varying, in relation to the low potential, excitation signal forsynchronizing operation of the fingerprint sensor; and an excitationsignal output for output of the excitation signal from the fingerprintsensor, wherein the method comprises the steps of: conductivelyconnecting the excitation signal output to a reference potential of adevice comprising the fingerprint sensor resulting in a variation overtime, in relation to the reference potential, of the low potential andthe high potential; providing a substantially constant potentialdifference between the high potential and the low potential; andacquiring from each of the sensor elements, while providing theexcitation signal, the signal indicative of the capacitive couplingbetween the sensing structure and the finger placed on the surface ofthe fingerprint sensor.

In summary, the present invention relates to a fingerprint sensingsystem comprising a sensor array with a plurality of sensing structuresand read-out circuitry connectable to each of the sensing structures,and power supply circuitry arranged to provide to the read-out circuitrya substantially constant supply voltage being a difference between ahigh potential and a low potential. The fingerprint sensing system isconfigured in such a way that the low potential and the high potentialare variable while substantially maintaining the supply voltage, and theread-out circuitry is connectable to each of the sensing structures insuch a way that a variation in the low potential and the high potentialwhile substantially maintaining the supply voltage results in a changeof the charge carried by a sensing structure connected to the read-outcircuitry. The change in charge is indicative of a capacitive couplingbetween the sensing structure and the finger.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showing anexample embodiment of the invention, wherein:

FIG. 1 schematically illustrates an application for a fingerprintsensing system according to an example embodiment of the presentinvention;

FIG. 2 schematically shows the sensor array comprised in the fingerprintsensing system in FIG. 1;

FIG. 3 is a schematic functional illustration of the fingerprint sensingsystem according to an example embodiment of the present invention wheresensing structures and a fingerprint ridge are shown in cross-section;

FIG. 4 schematically illustrates a method for reducing unwantedcapacitive driving of the finger;

FIG. 5 is a schematic illustration of a first embodiment of thefingerprint sensing system according to the present invention;

FIG. 6 is a schematic illustration of a second embodiment of thefingerprint sensing system according to the present invention;

FIGS. 7a-c schematically illustrate a third embodiment of thefingerprint sensing system according to the present invention;

FIGS. 8a-b schematically show example embodiments of the power supplycircuitry comprised in the fingerprint sensing system;

FIG. 9 schematically illustrates an advantageous timing of read-out andcommunication with the sensor array in the fingerprint sensing systemaccording to embodiments of the present invention; and

FIGS. 10a-b schematically illustrate example configurations of sensingelements comprised in embodiments of the fingerprint sensing systemaccording to the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the present detailed description, various embodiments of thefingerprint sensing system and method according to the present inventionare mainly described with reference to a fingerprint sensing system inwhich the low potential and the high potential of the supply voltage forpowering the sensor array are actively controlled to vary insynchronization with the read-out from the individual sensing structuresin the sensor array. Moreover, the sensor array is illustrated as atouch sensor dimensioned and configured to acquire a fingerprintrepresentation from a stationary finger.

It should be noted that this by no means limits the scope of the presentinvention, which equally well includes, for example, a fingerprintsensing system in which the potential of the finger is actively variedand the low potential and the high potential of the supply voltage forpowering the sensor array follow the variations of the finger potential.Other sensor array configurations, such as a so-called swipe sensor (orline sensor) for acquiring a fingerprint representation from a movingfinger, are also within the scope of the present invention as defined bythe appended claims.

FIG. 1 schematically illustrates an application for a fingerprintsensing system according to an example embodiment of the presentinvention, in the form of a mobile phone 1 with an integratedfingerprint sensing system 2. The fingerprint sensing system 2 may, forexample, be used for unlocking the mobile phone 1 and/or for authorizingtransactions carried out using the mobile phone, etc.

FIG. 2 schematically shows the capacitive fingerprint sensor 4 comprisedin the fingerprint sensing system 2 in FIG. 1. As can be seen in FIG. 2,the fingerprint sensor 4 comprises a sensor array 5, a power supplyinterface 6 and a communication interface 7. The sensor array 5comprises a large number of pixels 8 (only one of the pixels has beenindicated with a reference numeral to avoid cluttering the drawing),each being controllable to sense a distance between a sensing structure(top plate) comprised in the pixel 8 and the surface of a fingercontacting the top surface of the sensor array 5. In the enlargedportion of the sensor array 5 in FIG. 2, a first set of pixels aremarked ‘S’ for sensing, a second group of pixels are marked ‘N’ fornon-sensing, and a third group of pixels are marked D′ for driving.

The power supply interface 6 comprises first 10 a and second 10 bcontact pads, here shown as bond pads, for connection of a supplyvoltage V_(supply) to the fingerprint sensor 4.

The communication interface 7 comprises a number of bond pads forallowing control of the fingerprint sensor 4 and for acquisition offingerprint data from the fingerprint sensor 4.

The fingerprint sensor 4 in FIG. 2 may advantageously be manufacturedusing CMOS technology, but other techniques and processes may also befeasible. For instance, an insulating substrate may be used and/orthin-film technology may be utilized for some or all process stepsneeded to manufacture the fingerprint sensor 4.

The functional configuration of the fingerprint sensing system 2 in FIG.1 will now be described in more detail with reference to FIG. 3.

To aid the understanding of the reader, FIG. 3 is a hybrid of aschematic cross-section view and a functional block diagram. Referringto FIG. 3, a fingerprint ridge 12 contacts the top surface 13 of adielectric layer 14 protecting the sensing structures 15 a-c. Althoughnot explicitly shown in FIG. 3, the fingerprint ridge 12 is of coursesurrounded by fingerprint valleys where the finger surface is spacedapart from the top surface 13 of sensor array 5. Connected to each ofthe sensing structures 15 a-c is a corresponding pixel circuit 16 a-c.The pixel circuits 16 a-c are powered by a power source 17 via first 18a and second 18 b power lines. As is indicated in FIG. 3, each pixelcircuit 16 a-c has a control input 19 a-c and an output 20 a-c forproviding a signal indicative of the capacitive coupling between thecorresponding sensing structure 15 a-c and the finger 12.

In the example embodiment of FIG. 3, each pixel circuit 16 a-c iscontrollable between a sensing state, a non-sensing state, and a drivingstate by providing corresponding control signals to the respectivecontrol inputs 19 a-c.

When a pixel circuit, say 16 b, is in the sensing state, so that thepixel is a sensing pixel (denoted by ‘S’ in FIG. 2), it is configured toprovide, at its output 20 b, a sensing signal indicative of a change ofa charge carried by the sensing structure 15 b to which it is connected.The change of the charge is in turn indicative of the capacitivecoupling between the sensing structure (plate) 15 b and the finger 12.The capacitive coupling is an indication of the distance between the topsurface 13 of the sensor 4 and the finger surface.

When a pixel circuit, say 16 _(a) and 16 c, is in the non-sensing state,the pixel is a non-sensing pixel (denoted by ‘N’ in FIG. 2). The pixelcircuit 16 a,c is then in such a state that no sensing signal is output.Other pixel circuits (not shown in FIG. 3) may be in a driving state sothat their pixels are driving pixels (denoted by D′ in FIG. 2). Adriving pixel circuit provides a signal to the sensing structure (plate)to which it is connected.

During a sensing operation, the low potential V_(L) and the highpotential V_(H) defining the supply voltage V_(supply) are allowed tovary as is schematically indicated in FIG. 3. Accordingly, each of thepixel circuits 16 a-c will be powered using a substantially constantsupply voltage V_(supply), which moves up and down in potential as isschematically indicated in FIG. 3.

The pixel circuits 16 a-c are connected to their respective sensingstructures 15 a-c in such a way that a variation, in relation to areference potential of the electronic device 1 in which the fingerprintsensing system 2 is included, in the low potential V_(L) and the highpotential V_(H) while substantially maintaining the supply voltageV_(supply) results in a variation of the potential also on therespective sensing structures 15 a-c connected to the pixel circuits 16a-c.

It should be noted that the description provided in connection with FIG.3 focuses on the supply of power to the sensor array and that the exactcircuit and/or physical layout of each sensor element/pixel is notdescribed in detail.

As will be appreciated by the skilled person, many different circuitlayouts and/or physical layouts will be within the scope of the presentinvention. One example of a suitable pixel layout is provided in U.S.Pat. No. 7,864,992, which is hereby incorporated by reference in itsentirety. This exemplary pixel configuration will also be describedfurther below with reference to FIGS. 10a -b.

In some embodiments, it may desirable to reduce the number of pixelsthat exhibit a varying potential on its respective sensing structure toreduce capacitive drive of the finger. This may be achieved by allowingonly a segment of the sensor array to vary in potential, or by using thedriving capability that the pixel circuits may have.

In embodiments where all pixel circuits are connected to the same powersupply lines 18 a-b, the reference potential (local ground) for eachpixel circuit will follow the potential variations, in relation to areference potential of the electronic device 1 in which the fingerprintsensing system 2 is included, on the power supply lines 18 a-b. If allsensing structures of the sensor array 5 are allowed to follow thepotential variation of the power supply lines 18 a-b, the sensingstructures may collectively drive the finger 12 capacitively. Thiscapacitive drive of the finger 12 may reduce the desired potentialdifference between the finger and the sensing structure(s) 15 b of thecurrently sensing pixel(s) (‘S’).

An exemplary method of reducing the effect of this unwanted capacitivedrive of the finger 12 is to control selected pixel circuits to theirdriving state and to provide a drive signal V_(drive) to the sensingstructures of the driving (‘D’) pixels, which drive signal V_(drive) issubstantially in anti-phase with the variation of the potential V_(s)induced by the varying low V_(L) and high V_(H) potentials.

The resulting potential V_(N) on the sensing structures of the driving(‘D’) pixels may, depending on the amplitude of the variations of thelow V_(L) and high V_(H) potentials, be substantially constant or, as isschematically illustrated in FIG. 4, vary with a lower peak-to-peakamplitude than the potential V_(s) on the sensing structure(s) 15 b ofthe sensing (‘S’) pixel(s).

In the following, three example embodiments of the fingerprint sensingsystem according to the present invention will be described withreference to the schematic functional illustrations provided in FIGS. 5,6, and 7 a-c.

Starting with the first example embodiment, FIG. 5 schematically shows afingerprint sensing system 2 comprising a fingerprint sensor 4,processing circuitry 25, here in the form of a microprocessor, and powersupply circuitry 26.

The fingerprint sensor 4 comprises, as was also described above withreference to FIG. 2, a sensor array 5, a power supply interface 6 and acommunication interface 7. The communication interface 7 is, in thepresent example, provided in the form of an SPI-interface (SerialPeripheral Interface).

The microprocessor 25 acquires fingerprint data from the fingerprintsensor 4 and may process the fingerprint data as required by theapplication. For instance, the microprocessor 25 may run a fingerprintmatching (verification and/or identification) algorithm.

The power supply circuitry 26 comprises a sensor voltage source 27configured to output the supply voltage V_(supply) and a pulsegenerator, here a square wave pulse generator 28, for modulating the lowpotential side of the sensor voltage source 27 in relation to areference potential DGND of the electronic device 1 in which thefingerprint sensing system 2 is included.

As an alternative to a square wave signal, the pulse generator maygenerate any other suitable pulse shape, such as a sine wave or a sawtooth signal etc.

The sensor voltage source 27 may be provided in the form of a constantvoltage source, such as a battery, dedicated to supplying power to thefingerprint sensor 4. Alternatively, the sensor voltage source 27 maycomprise isolation circuitry for at least partly isolating a voltagesource from the fingerprint sensor 4.

Example configurations of the sensor voltage source 27 in which itcomprises isolation circuitry will be described further below withreference to FIGS. 8a -b.

In this first embodiment, the fingerprint sensing system 2 does notcomprise any external structure for electrically contacting the finger12. For the fingerprint sensing system 2 in FIG. 5, it may therefore bebeneficial to reduce unwanted capacitive drive, for example as describedabove with reference to FIG. 4.

As is schematically indicated using a box with dashed line boundaries,the fingerprint sensing system 2 may optionally additionally includeisolation circuitry 29 for providing galvanic isolation or levelshifting between the sensor array 5 and the microprocessor 25. Throughthe use of isolation circuitry 29, the microprocessor 25 is allowed towork independently of the varying, in relation to DGND, referencepotential V_(L) of the sensor array 5.

As was mentioned in the Summary section, the isolation circuitry 29 canbe implemented in many different ways known to the skilled person. Forinstance, the isolation circuitry 29 may be implemented using componentssuch as opto-couplers, coils and/or capacitors. Although shown here asseparate circuitry, it should be understood that the isolation circuitry29 may be integrated with the fingerprint sensor 4 or the microprocessor25.

FIG. 6 schematically shows a second embodiment of the fingerprintsensing system 2 in FIG. 1, that differs from the first embodimentdescribed above with reference to FIG. 5 in that it additionallycomprises a conductive bezel or frame 30 arranged adjacent to thefingerprint sensor 4 to allow the finger 12 to touch the bezel 30 whenthe finger 12 is placed on the fingerprint sensor 4.

Moreover, the pulse generator 28 is connected between the voltage source27 and the bezel 30 instead of between the voltage source 27 and groundas in the first embodiment of FIG. 5.

Through the connection between the finger 12 and the conductive bezel30, the effect of any unwanted capacitive drive can be mitigated.

Optionally, the conductive bezel 30 may be grounded which may beadvantageous for handling ESD. When the fingerprint sensing system 2 isin use, the conductive bezel 30 will be in conductive contact with theuser's finger. The finger (body) of the user is a much larger conductivebody than the fingerprint sensor 4, which means that the fingerpotential will be largely unaffected by the pulse provided by the pulsegenerator 28, and the sensor ground potential V_(L) will swing up anddown as is schematically indicated in FIG. 6.

FIGS. 7a-c schematically illustrate a third embodiment of thefingerprint sensing system 2 in FIG. 1, that differs from the firstembodiment described above with reference to FIG. 5 in that the powersupply input to the fingerprint sensor 4 is not actively varied inrelation to the device ground DGND, and in that the fingerprint sensor 4additionally comprises excitation signal generating circuitry (not shownin FIG. 7a ) for synchronizing the measurement and read-out (such assampling and A/D-conversion) of signals indicative of the distancebetween sensing structures comprised in the sensor array 5 and thefinger surface. The fingerprint sensor 4 in FIG. 7a further comprises anexcitation signal output 32 for output of the excitation signal TX fromthe fingerprint sensor 5.

Between the voltage supply, providing a constant supply potential V_(cc)related to device ground DGNC, and the voltage supply interface 6 of thefingerprint sensor 4, there is provided voltage isolation circuitry 33making the potential levels on the fingerprint sensor side independentof the potential levels on the device side (left in FIG. 7a ). Thevoltage isolation circuitry may be realized in many different ways knownto one skilled in the relevant art. One example of suitable circuitrywill be described below with reference to FIG. 8b . Other suitablecircuitry include various transformer solutions or switched networks.

Operation of the fingerprint sensing system 2 in FIG. 7a will now bedescribed with reference to the timing diagrams in FIGS. 7b -c.

The timing diagrams in FIGS. 7b-c both illustrate electric potential asa function of time, but in relation to different reference potentials.

In FIG. 7b , an exemplary excitation signal TX is shown in relation tothe reference potential V_(L) of the fingerprint sensor 4 (also referredto as sensor ground). As can be seen in FIG. 7b , the excitation signalTX is a square wave signal varying over time between a low level V_(L)and a high level V_(H).

FIG. 7c shows the same excitation signal TX as in FIG. 7b , but now inrelation to device ground DGND. Since, as is schematically illustratedin FIG. 7a , the excitation signal output 32 is conductively connectedto the supply potential V_(cc) of the device 1, which is a constantpotential in relation to the device ground DGND, and the fingerprintsensor system 2 is configured such that the potential levels of thefingerprint sensor 4 can swing up and down in potential, the excitationsignal TX will exhibit a substantially constant potential in relation todevice ground. Consequently, the sensor ground V_(L) will vary over timein relation to device ground DGND as is schematically indicated in FIG.7 c.

FIG. 8a schematically shows a first embodiment of the sensor voltagesource 27 comprising a voltage source 37, such as a battery, andisolation circuitry 38. The isolation circuitry 38 is configured toprovide galvanic isolation between its input side, to which the voltagesource 37 is connected, and its output side, that is connected to thesensor 4 as shown in FIGS. 5 and 6. The isolation circuitry 38 in FIG.8a may be implemented in many different ways known to the skilledperson. For instance, the isolation circuitry 38 may be implementedusing components such as opto-couplers, coils and/or capacitors.Although shown here as separate circuitry, it should be understood thatthe isolation circuitry 38 may be integrated in the fingerprint sensor4.

FIG. 8b schematically shows a second embodiment of the sensor voltagesource 27 comprising an input 39, a diode 40 and a capacitor 41. Thediode 40 is connected between the input 39 of the sensor voltage source27 and the high potential input of the fingerprint sensor 4, and thecapacitor 41 is connected between the low potential V_(L) and the highpotential V_(H) of the supply voltage for the fingerprint sensor 4. Theinput 39 is for connection of the supply voltage V_(cc) of the devicecomprising the fingerprint sensing system 2 as was described above withreference to FIGS. 7a -c.

The isolation circuit in FIG. 8b does not provide full galvanicisolation between the supply voltage V_(cc) and the fingerprint sensor4, but enables the provision of time-varying low potential V_(L) andhigh potential V_(H) to the fingerprint sensor 4 even though the supplyvoltage (or rather supply potential) V_(cc) remains constant.

It should be noted that the voltage source 37 in FIG. 8a as well as thesupply voltage V_(cc) provided on the input 39 in FIG. 8b may typicallybe used for providing power to additional devices, such as themicroprocessor 25 and/or other components of the device comprising thefingerprint sensing system 2.

Above, the use of isolation circuitry 29 was discussed as a means forenabling acquisition of fingerprint data from a sensor array with avarying (swinging) reference potential. Alternatively, the fingerprintdata from the sensor array 5 may be acquired by the microprocessor 25without the use of isolation circuitry 29 by scheduling read-out fromthe pixels 8 and acquisition of the fingerprint data from the sensorarray 5 in different time slots.

This will now be described with reference to the timing diagram in FIG.9, where each time slot 35 a-d for sensor read-out is followed by a timeslot 36 a-d for acquisition of fingerprint data from the sensor array.Signals from all pixels may be sampled and processed (for exampleconverted to digital) during each time slot 35 a-d for sensor read-out,or a segment of the sensor may be sampled and processed during each timeslot 35 a-d for sensor read-out. In the latter case, fingerprint datacorresponding to the segment that is sampled and processed during aparticular time slot 35 a-d for read-out is acquired during thesubsequent time slot 36 a-d for acquisition of fingerprint data.

During the time slots 35 a-d for sensor read-out, the power supplycircuitry 26 is controlled to allow the low V_(L) and high V_(H)potentials defining the supply voltage V_(supply) to vary, and duringthe time slots 36 a-d for fingerprint data acquisition, the power supplycircuitry 26 is controlled to keep the low V_(L) and high V_(H)potentials constant. In particular, the power supply circuitry 26 may becontrolled to set the low potential V_(L) to the same potential as theground potential of the microprocessor 25 (device ground DGND).

An example configuration of the sensing elements 8 comprised inabove-described embodiments of the fingerprint sensor 4 will now bedescribed with reference to FIGS. 10a -b.

As can be seen in FIG. 10a , the sensing elements are formed in a layerstructure comprising three conductive layers; a conductive layer M3 atthe top, a conductive layer M2 in the middle and a lower conductivelayer M1, with first 51, second 52, and third 53 layers of an insulatingdielectric material under the respective conductive layers M3, M2, M1.Examples of materials for the conductive layers are typically copper,aluminum and doped polycrystalline silicone. Examples of materials forthe insulating layers are typically SiO2, SiN, SiNOx and spin-on glass.

In addition, the layered structure used to form the sensing elements 3may comprise a fourth layer P2 (second polysilicon) constituted by anelectrically conducting layer which is kept at a certain analog voltagepotential AV_(dd) in relation to sensor ground V_(L). Further, there isa fifth layer P1 (first polysilicon) that is also constituted by anelectrically conducting layer which is kept at sensor ground potentialV_(L), working as an electric shielding. Under each one of these layersP2, P1 there are fourth 63 and fifth 64 layers of an insulatingdielectric material. At the bottom, there is a semi conductive substratelayer D1 comprising active components such as the charge amplifiers 54.The conductive layers P2, P1 as well as the lower conductive layer M1described above, may for example be used for routing of electricalconnections, resistors and electrical shielding. One of the conductivelayers P2, P1 may also be used to form the lower electrode 55 of eachsensing element 8 instead of the second metal layer M2.

The sensing element 8 shown in FIG. 10a comprises a sensing structure 15b formed in the top conductive layer M3. The sensing structure 15 b isconnected to a sensing element circuit 16 b comprising a chargeamplifier 54, a lower electrode 55, a reset switch 56, andsample-and-hold circuitry 65.

As can be seen in FIG. 10a , the sensing structure 15 b is connected tothe negative input terminal 58 of the charge amplifier 54. The positiveinput terminal 59 of the charge amplifier 54 is connected to the sensorground potential V_(L). Hence, by means of the charge amplifier 54, thecorresponding sensing structure 15 b is virtually grounded (sensorground), since the voltage over the input terminals 58, 59 of the chargeamplifier 54 is almost zero. Depending on the circuit implementation ofthe charge amplifier there may be a small substantially constantvoltage, such as the gate voltage of a CMOS transistor, between thenegative 58 and positive 59 input terminals of the operationalamplifier.

As can also be seen in FIG. 10b , each sensing structure 15 b may besurrounded by a shield frame 60 formed in the top conductive layer M3,where the shield frame 60 is connected to the sensor ground potentialV_(L) as a conductive shielding to prevent lateral parasiticcapacitances between adjacent sensing structures 15 b, thus preventingor at least reducing so-called crosstalk between the sensing elements 8.The shield frame 60 may also be connected to another suitable potential.

Further, referring again to FIG. 10a , there is a protective dielectriclayer 14 covering each of the sensing structures 15 b, to protect thesensing elements 8 from ESD (Electrostatic Discharge) and external wear.A finger 12 that comes into the vicinity of the upper surface of theprotective layer 14 gives rise to a capacitance C_(finger) between thefinger 12 and the sensing structure 15 b.

As can be seen in FIG. 10a , the lower electrode 55 comprised in thesensing element circuit 16 b is formed in the middle conductive layerM2. The lower electrode 55 is connected to an output terminal 20 b ofthe charge amplifier 54. There is a feedback capacitance C_(ref) formedbetween the sensing structure 15 b and each lower electrode 55, whichfeedback capacitance C_(ref) is connected between the negative inputterminal 58 of the charge amplifier 54 and the output terminal 20 b.

An auxiliary lower electrode 55 a is also formed in the middleconductive layer M2, adjacent to the lower electrode 55. The auxiliarylower electrode 55 a is connected to the sensor ground potential V_(L)and used as an extra shielding, since the lower electrode 55 maytypically have a smaller area than the sensing structure 15 b.

The lower electrode 55 may be configured to achieve the desired gain forthe sensor element circuit 16 b. In particular, the size of the lowerelectrode 55 may be suitably selected, since the gain depends on thefeedback capacitance C_(ref), which in turn is dependent on the physicaldimensions of the sensing structure 15 b, the lower electrode 55, andthe first insulating layer 51. The size of the auxiliary lower electrode55 a may be adjusted so as to fit beside the lower electrode 55.

As described above, swinging the sensor ground potential V_(L) inrelation to the potential of the finger 12 will result in a change inthe voltage between each sensing structure 15 b and the finger 12, whichwill in turn result in a change of the charge carried by the sensingstructures 15 b.

The change of charge that is carried by the sensing structure 15 b isproportional to the capacitance C_(finger) between the skin and thesensing structure 15 b. As the sensing structure 15 b is virtuallygrounded in relation to sensor ground V_(L), its charge is transferredby the charge amplifier 54 to the lower electrode 55. We may thencalculate the voltage output from the charge amplifier 54 as:

U _(out)=(C _(finger) /C _(ref))U _(in)

The output voltage U_(out) is sampled by the sample-and-hold circuitry65, preferably using correlated double-sampling to remove the lowfrequency component of the common mode noise.

The sample-and-hold circuitry 65 is controlled by a control signal andoutputs the pixel signal S_(pixel) indicative of the capacitive couplingbetween sensing structure 15 b and finger 12 to an analog-to-digitalconverter (not shown).

The person skilled in the art realizes that the present invention by nomeans is limited to the example embodiments described above. Forexample, the read-out circuitry and the driving circuitry may beprovided as separate circuits.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasured cannot be used to advantage.

What is claimed is:
 1. A fingerprint sensing system comprising: a sensor array having: a plurality of sensing structures; and read-out circuitry connected to each of said sensing structures for providing sensing signals; and power supply circuitry arranged to provide to said read-out circuitry a substantially constant supply voltage being a difference between a high driving voltage potential and a low driving voltage potential, wherein: the fingerprint sensing system is configured in such a way that, during operation of said fingerprint sensing system, said low driving voltage potential and said high driving voltage potential oscillate in phase in relation to a reference potential of a device comprising said fingerprint sensing system, while substantially maintaining said supply voltage.
 2. The fingerprint sensing system according to claim 1, wherein said power supply circuitry is configured to provide a first time-varying potential, varying in relation to said reference potential, as said low driving voltage potential and a second time-varying potential as said high driving voltage potential.
 3. The fingerprint sensing system according to claim 1, wherein said power supply circuitry comprises isolation circuitry having an input side for connection to a voltage source and an output side connected to said sensor array, said isolation circuitry being configured to prevent current to flow from said output side to said input side, to allow for an output potential on said output side being different from an input potential on said input side.
 4. The fingerprint sensing system according to claim 3, wherein said isolation circuitry is configured to provide galvanic isolation between said input side and said output side.
 5. The fingerprint sensing system according to claim 1, wherein said sensor array further comprises driving circuitry connectable to each of said sensing structures and controllable to change a potential, in relation to said low driving voltage potential, of a sensing structure connected to said driving circuitry.
 6. The fingerprint sensing system according to claim 1, further comprising processing circuitry connected to said sensor array via a communication interface for acquiring fingerprint data from said sensor array.
 7. The fingerprint sensing system according to claim 6, further comprising isolation circuitry for providing galvanic isolation between said sensor array and said processing circuitry.
 8. The fingerprint sensing system according to claim 6, wherein said low driving voltage potential and said high driving voltage potential are variable in relation to a reference potential of said processing circuitry.
 9. The fingerprint sensing system according to claim 6, wherein said power supply circuitry is configured to keep each of said low driving voltage potential and said high driving voltage potential substantially constant, in relation to said reference potential of the processing circuitry, during time periods when said processing circuitry acquires fingerprint data from said sensor array.
 10. The fingerprint sensing system according to claim 1, wherein said fingerprint sensing system further comprises an electrically conductive structure being arranged to be in electrical contact with said finger when said finger is placed on said surface of said sensor array.
 11. The fingerprint sensing system according to claim 1, wherein said sensor array comprises: excitation signal generating circuitry for generating a time-varying, in relation to said low driving voltage potential, excitation signal for synchronizing operation of said read-out circuitry; and an excitation signal output for output of said excitation signal from said sensor array, wherein said excitation signal output is conductively connected to said reference potential, thereby forcing said low driving voltage potential and said high driving voltage potential to vary over time in relation to said reference potential.
 12. The fingerprint sensing system according to claim 1, wherein: said fingerprint sensing system further comprises a conductive finger contacting structure arranged adjacent to said sensor array for electrically contacting said finger; and said sensor array comprises: excitation signal generating circuitry for generating a time-varying, in relation to said low driving voltage potential, excitation signal for synchronizing operation of said read-out circuitry; and an excitation signal output for output of said excitation signal from said sensor array, wherein said excitation signal output is conductively connected to said finger contacting structure, thereby forcing said low driving voltage potential and said high driving voltage potential to vary over time in relation to a potential of said finger.
 13. An electronic device comprising: the fingerprint sensing system according to claim 11, and processing circuitry configured to: acquire a representation of said fingerprint pattern from the fingerprint sensing system; authenticate a user based on said representation; and perform at least one user-requested process only if said user is authenticated based on said representation, wherein the excitation signal output of the sensor array comprised in said fingerprint sensing system is conductively connected to the reference potential of the electronic device.
 14. A method of operating a fingerprint sensor comprising: a sensor array comprising a plurality of sensor elements; and a power supply interface having a low driving voltage potential input and a high driving voltage potential input for providing power to said sensor array, wherein said method comprises the steps of: providing a first time-varying driving voltage potential, in relation to a reference potential of a device comprising said fingerprint sensor, to said low driving voltage potential input of the power supply interface and a second time-varying driving voltage potential, in relation to a reference potential of a device comprising said fingerprint sensor, to said high driving voltage potential input of the power supply interface, said first time-varying driving voltage potential and said second time-varying driving voltage potential oscillating in phase in such a way that a difference between said second time-varying driving voltage potential and said first time-varying driving voltage potential is a substantially constant voltage; and acquiring a sensing signal from each of said sensor elements, while providing said first time-varying driving voltage potential and said second time-varying driving voltage potential.
 15. The method according to claim 14, further comprising the step of: providing, through a communication interface comprised in said fingerprint sensor, a signal indicative of a fingerprint pattern of said finger, said signal indicative of the fingerprint pattern of said finger further encoding error correction data for enabling error correction. 